Using intermediate nodes of graph as input to function

[zxie]

Suppose we have function([x], y) where we have z = f(x), y = g(z). We then want to compile function([z], y). Currently two issues:

1. z is a Result and not an Argument node
2. create_execution_graph processes all ancestors of y (including x) when trying to compile function([z], y); it should be blocked at provided input z.